Chromium plating



United States Patent-O CHROMIUMv PLATING Jesse E. Stareck, Royal Oak, and Ronald Dow, Detroit, Micln, assignors, by mesne assignments, to Metal & Thermit Corporation, New York, N. Y., corporation of New Jersey Application August 12, 1954, Serial No. 449,457

16 Claims. (Cl..20451) This invention relates to chromium plating using a chromic acid plating bath and particularly to a method for producing crack-free deposits of chromium.

In copending application, Serial No. 356,188, filed May 20, 1953, now Patent 2,686,756, issued August 17, 19.54, of which the present application is a continuation-in-part, there is described and claimed a method of producing crack-free chromium plate by maintaining, over a range of chromic acid concentration, particular and well defined amounts of catalyst acid radicals whose concentrations are not only self-regulated but also controlled by means of non-catalytic suppressor compounds, and by maintaining the temperature of the bath solution above a particular minimum value.

The present invention has the. same purpose as the said cop'ending application and utilizes similar means for accomplishing the purpose except that the catalyst acid radical content of the bath solution isv not controlled by non-catalytic suppressor compounds. In the present method the bath solution comprises chromic acid, and two soluble catalyst-supplying compounds, namely, a sulfate radical bearing compound and a silicofiuoride radical bearing compound. After the bath solution has been initially made up to the required concentrations, the catalyst content is maintained by dissolving catalyst-supplying compounds in the bath as required. Compounds of varying solubility may be used, including those readily soluble in the bath solution, those sparingly soluble therein, and those of intermediate solubility, all of which compounds are intended herein as: being soluble compounds. Specific sulfate-supplying compounds include readily soluble compounds like sulfuric acid, sodium sulfate, potassium sulfate, chromium sulfate, sparingly soluble compounds like strontium sulfate and' compounds of intermediate solubility-such as calcium sulfate. Specific silicofiuoride-supplying compounds include readily soluble compounds like hydrofiuosilicic acid and magnesium silicofiuoride, sparingly soluble compounds like potassium silicofiuoride, and compounds of intermediate solubility dissolved silicofluoride from 1.0 to 11.4 g./l., with the sum of the two catalysts lying in the range of 1.5 to

11.7 g./l. Non-catalytic suppressor compounds are kept out of the bath, so that the foregoing concentrations of dissolved catalysts are values not affected by suppressor compounds.

The sum of the dissolved sulfateand silicofluoride varies with the dissolved CrOs in the manner defined by the area ABCD of the-graph shown in the accompanyingdrawing. Asnoted in the drawing, the graph shows the variation of the chromic-acid concentration in grams per 2,787,589 Patented Apr. 1957 2 liter with the concentration, in grams per liter, of the; sum of dissolved sulfate and silicofluori'de. As the chromic acid varies from 200 to 900 g./l., the catalyst content varies from a low of 1.5 g./l. to a high of 11.7 g./l. Within the area ABCD, the plate that is deposited is crack-free and has the other advantages described hereinafter. At catalyst concentrations that lie above the curve DC, the plate is cracked, and at catalyst concentrations below the curve AB, the plate is nodular and unsuitable. The above described variation of the chromic acid with the sum of the sulfate and silicofluoride may also be shown by the following table, which also shows how the sulfate and silicofiuoride' concentrations may individually vary:

200 1. 5-2. 3 0. 3-0. 8 1.0-2. 0 300 2.0-3. 7 0. 3-1. 5 1. U-3. 4 400' 2. 5-5. 1 0. 3-2. 2 1. O-4. 8 500 3. O-Gl 3 0-3-2. 8 1. 0-6. 0 600 3. 5-7. 6 0. 3-3. 3 1. 0-7. 3 700 4. 0-8. 9 0. 3-3. 9 1. O-8. 6 800 1. 5-10. 5 0. 3-4. 3 1. 0-10. 2 9,00 5. 0-11. 7 0. 3-5. 5 l. ()-1 1. 4

Preferably, the sum of the dissolved sulfate. and silicofluoride is in the range of 1.9 to 7.6 g;./l. The preferred concentration of dissolved sulfate may vary from 0.3 to 4.5- g./l. and that of dissolved, silicofluoride from 1 .5 to 6.3 g./l. with the preferred sum of the two catalysts lyingin the range of 1.9 to 7.6 g./l. The preferred:- variation of the sum of the dissolved catalyst radicals with the C103 concentration, therefore, is as defined by the area JKLM of the graph, and is also set forth in the following table:

TABLE .2

CrOa, SO4+SiFa, 804, 32/1. SiFu, g./l. g./1. g./l.

200 1. 9-2.2 0.3-0.7 1.5-1.9 300 2.4-2.9 0. 4-0.9 1. 62.2 '400 2. 9-3. 7 0. 5-1. 2' 1.8-2.6 500 3.4-4. 5 0. 6-1.6 j 2.1-3.1 600 3. 9-5. 3 0. 7-2. 1 2. 5-3. s 700 4. 4-6. 1 0.9-2.8 3. 0-4. 5 800 4. 9-6. s 1. 1-3. 6 a. 5-5. 2 900 5. 4-7; 6 1. 3-4. 5 4. 1-6. a

it is desirable that the concentration of sulfate should exceed that of silicofiuoride in the area EBF of the graph and that the concentration of; silicofiuoride. should exceed that of sulfate in the area GHCD. The variations represented by these areas are also illustrated by the following The plating bath may be made up by adding a mixture of the ingredients to water and stirringuntil equilibrium is obtained. The bath may also be maintained in this way. The chromic acid content maybe supplied by adding chromic acid as such to the bath. The catalyst acid radical content is maintained by dissolving catalyst-supplying compounds in the bath as required and in amounts equivalent to the amounts of catalyst radicals to be replaced. Compounds that are readily soluble may require more frequent addition than those sparingly soluble since, as noted below, the latter may sometimes be added in excess so as to saturate the bath and to provide an undissolved residue. The bathanalysis should be available when required in order to determine when additions are necessary. When solutions of readily soluble catalystsupplying compounds, such as sulfuric acid and hydro fluosilicic acid, are used to replenish the bath, they should, of course, be of known strength. With compounds like strontium sulfate and potassium silicofluoride, which are sparingly soluble, it is possible under certain conditions,

such as high chromic acid concentration, to add an excess of either or both compounds so that the bathpbecomes saturated, leaving an undissolved quantity of compound or compounds on the floor of the bath which will gradually dissolve to replace catalyst radical that is lost; furthermore, it is thus possible to provide a catalyst concentration within the required range using only two sparingly soluble compounds such as strontium sulfate and potassium silicofiuoride, each present in excess. These latter two compounds can be used alone at lower chromic acid concentration by adding one or other or both in amounts that do not saturate the bath, that is, amounts that completely dissolve. Sparingly soluble compounds may also be employed in conjunction with one or more other more soluble compounds. As will be understood, readily soluble compounds are useful throughout the chromic acid range, either alone, as in the case of sulfuric and by drotluosilicic acids, or in combination with one or more other readily or less soluble compounds. Similarly, compounds of intermediate solubility may be used alone or in combination with other compounds.

In making up or replenishing the bath, the use of a catalyst-supplying compound which would act as a suppressor in' regard to another catalyst-supplying compound is omitted; for examplc,:if potassium silicofluoride is present in the bath or is used to supply silieofiuoride, then potassium sulfate is not used as the source of supply of sulfate, :1 salt like sodium sulfate being used instead.

The plating method comprises passing current in the r range of 0.5 to 8, preferably 1 to 3, amperes per square inch (a. s. i.) from an anode to an article cathode immersed in the bath The bath is operated at a minimum temperature of F., it having been found that below this'temperature the plate that is produced is cracked. The upper temperature may extend to the boiling point of the bath but preferably is about 180 F. A preferred temperature range is to 160 F. Generally, with higher temperatures the concentrations of dissolved CrOs, sulphate and silicofiuoride should be higher, and higher current densities are used.

The crack-free quality of the chromium plate produced by the present method is not only determinable visually but also by means of other tests, as set forth in said copending application. The crack-free plate of the invention is characterized by having a hardness of about 425 to about 825, usually 550 to 700, Knoop. The plate is relatively soft and ductile as compared with conventional chromium plate. It has a smooth texture, a satiny finish, and a dull, matte, white color. It is easy to polish or buff.

The invention may be illustrated by the following examples:

, Example 1 A solid mixturewzis prepared comprising 900 parts of chromic acid, 15.5 parts of potassium silicofluoride, and 8.5 parts of strontium sulfate, all parts by weight. The mixture was added to water to form a solution having 900 g./l. of C103, 6.0 g./l. of silicofluoride, and 1.7 g./l. of sulfate. Two steel rods of diameter were plated in the solution at F. and a current density of 2 a. s. i., one of the rods being plated for 3 hours and the other for 14 hours. In each case a smooth dull crack- -free chromium plate was deposited. The thickness of the plate deposited during the 3 hour run was 0.9 mil, while the thickness of the plate during the 14 hour run was 4.2 mils.

Example 2 A plating solution was made up containing 750 g./l. of CrOz, 15.0 g./l. sodium silicofluoride, 7.5 g./l. strontium sulfate, and 1.0 g./l. of hydrofluosilicic acid. The solution contained 12.2 g./l. of silicofiuoride and 2.5 g./l. of sulfate. Asteel rod, 4" long X in diameter, was plated in the solution at 6 a. s. i. for one hour at F. A dull smooth deposit of chromium was produced having a thickness of 0.95 mil and substantially free of cracks. Another steel rod, of the same size as the first one, was plated in the solution at 8 a. s. i. for one-half hour at F. The deposit was smooth, dull, and dark gray, had a thickness of 0.20 mil, and was free of cracks.

Example 3 A solution was made up having 750.0 g./l. of CrOa,

15.0 g./l. of sodium silicoiluoride, 7.5 g./l. strontium sulfate, and 0.5 g./l. of sulfuric acid. The solution contained 11.3 g./l. of silicofiuoridc and 3.0 g./l. of sulfate. A steel rod, 4" long x as" in diameter, was plated in the solution at 6 a. s. i. for one hour at 160 F. to produce a dull, smooth deposit of chromium of 1.2 mils thickness. The deposit was free of cracks. Another steel rod, 4 long x in diameter was plated in the solution at 8 p a. s. i. for one-half hour at l80 F. to produce a smooth,

dull, crack-free deposit of chromium of 0.40 mil thickness.

Example 4 An aqueous solution was prepared by dissolving CrOs, sulfuric acid, and hydrofiuosilicic acid in water to form a solution having 350 g./l. of chromic acid, 1.8 g./l. of silicofluoride, and 1.1 g./l. of sulfate. A steel mandrel /8" in diameter and 4" long was plated in the solution at 150 F. and a current density of 3 a. s. i. for 3 hours, there being deposited a smooth, dull white crack-free chromium plate having a thickness of 2.8 mils.

Deposits of varying thicknesses may be plated, ranging for example from a thickness just suflicient to cover the pores of the basis metal to any desired thicker deposit for which a demand may exist.

The invention is useful for articles made of any of a variety of basis metals, such as plain carbon steels, alloy steels including stainless steel, iron, cast iron, Zinc and zinc alloys, nickel and nickel alloys, copper and copper alloys, aluminum and aluminum alloys, tin and tin alloys, lead and lead alloys, etc. In general, the basis metal may be any metal, or an undercoat on a metal, that can be chromium plated.

In the light of the foregoing description, the following is claimed:

1. A method of electrodepositing crack-free, chromium plate on an article of metal which comprises essentially: passing current in the range of 0.5 to 8 amperes per square inch from an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of 140 F. to the boiling point of the bath, said bath comprising essentially 200 to 900 g./l. CrOs, atleast one soluble sulfate-containing compound in an amount sufiicient to produce a concentration of 0.3 to 5.5 Y

i g./l. of dissolved sulfate (50F) and at least one soluble silicoiluoride-containing compound in an amount suffidissolved silicofluoride (SiFs=), said bath being free of compounds acting to suppress the concentrations of dissolved sulfate and silicofluoride, the sum of said dissolved sulfate and silicofluoride being in the range of 1.5 to 11.7 g./l., and said sum of dissolved sulfate and silicofluoride varying with the CrOs concentration in the manner defined by the area ABCD of the graph shown in the accompanying drawing.

2. A method according to claim 1 in which the metal article that is chromium plated is an article of plain carbon steel.

3. A method according to claim 1 in which said sulfatecontaining compound is selected from the class consisting of sulfuric acid and soluble sulfate salts, and said siliccfiuoride-containing compound is selected from the class consisting of hydrofluosilicic acid and soluble silicofluoride salts.

4. A method according to claim 3 in which said sulfateand silicofiuoride-containing compounds are soluble sulfate and silicofluoride salts.

5. A method according to claim 3 in which said sulfateand silicofluoride-containing compounds are sulfuric and hydrofluosilicic acids.

6. A method according to claim 4 in which said sulfate and silicofiuoride salts are salts of different alkali metals, said alkali metals being selected from the group consisting of sodium and potassium.

7. A method of electrodepositing crack-free, chromium plate on a metal article which comprises essentially:

passing current in the range of 0.5 to 8 amperes per square inch from an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of 140 to 180 F., said bath comprising essentially 200 to 900 g./l. CrOs, a soluble sulfate-containing compound and a soluble silicofluoride-containing compound each in an amount sufficient to produce a concentration of 1.5 to 11.7 g./l. of the sum of dissolved sulfate (SO4=) and dissolved silicofluoride (SiFs=), said bath being free of compounds acting to suppress the concentrations of dissolved sulfate and silicofluoride, said sum of dissolved sulfate and silicofiuoride varying with the CrOa concentration in the manner defined by the area ABCD of the graph shown in the accompanying drawing, the concentration of sulfate exceeding that of silicofiuoride in the area EBF of said graph and the concentration of silicofiuoride exceeding that of sulfate in the area GHCD.

8. An improved method of producing a corrosion resistant chromium plated metal article comprising plating the article in an aqueous bath comprising essentially 200 to 900 g./l. C103 and two catalyst-supplying compounds in the form of a soluble sulfate-containing compound and a. soluble silicofluoride-containing compound, said latter compounds being present each in an amount sufiicient to produce, as catalysts, a concentration of 0.3 to 5.5 g./l. of dissolved sulfate (SO4=) and a concentration of 1.0 to 11.4 g./l. of dissolved silicofluoride (SiFs=), respectively, said bath being free of compounds acting to suppress the concentrations of dissolved sulfate and silicofluoride, the sum of said dissolved sulfate and silicofluoride being in the range of 1.5 to 11.7 g./l., said sum of dissolved sulfate and silicofiuoride varying with the C103 concentration in the manner defined by the area ACBD of the graph shown in the accompanying drawing; performing the plating at a bath temperature of to F. and a current density of 0.5 to 8 amperes per square inch, thereby producing on the article a crack-free deposit of chromium, and replenishing the catalyst content to maintain the above mentioned concentration ranges by adding at least one soluble catalyst-supplying compound to the bath.

9. A method according to claim 8 in which said sulfate-containing compound is selected from the class consisting of sulfuric acid and soluble sulfate salts, and said silicofluoride-containing compound is selected from the class consisting of hydrosilicofiuoric acid and soluble silicofiuoride salts.

10. A method according to claim 9 in which said sulfateand silicofiuoride-containing compounds are soluble sulfate and silicofiuoride salts.

11. A method according to claim 9 in which said sulfateand silicofiuoride-containing compounds are sulfuric and hydrofluosilicic acids.

12. A method according to claim 10 in which said sulfate and silicofiuoride salts are salts of different alkali metals, said alkali metals being selected from the group consisting of sodium and potassium.

13. A method according to claim 8 in which the catalyst content of the bath is replenished by adding a readily soluble catalyst-supplying compound.

14. A method according to claim 13 in which the catalyst-supplying compound is an acid.

15. A method according to claim 13 in which the catalyst-supplying compound is a salt.

16. A method according to claim 8 in which the catalyst content of the bath is replenished by adding a sparingly soluble catalyst-supplying compound.

References Cited in the file of this patent UNITED STATES PATENTS 2,640,021 Passal May 26, 1953 2,640,022 Stareck May 26, 1953 2,686,756 Stareck et al Aug. 17, 1954 

1. A METHOD OF ELECTRODEPOSITING CRACK-FREE, CHROMIUM PLATE ON AN ARTICLE OF METAL WHICH COMPRISES ESSENTIALLY PASSING CURRENT IN THE RANGE OF 0.5 TO 8 AMPERES PER SQUARE INCH FROM AN ANODE TO SAID ARTICLE AS A CATHODE IMMERSED IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATURE OF 140*F. TO THE BOILING POINT OF THE BATH, SAID BATH COMPRISING ESSENTIALLY 200 TO 900 G./. CRO3 AT LEAST ONE SOLUBLE SULFATE-CONTAINING COMPOUND IN AN AMOUNT SUFFICIENT TO PRODUCE A CONCENTRATION OF 0.3 TO 5.5 G./1. OF DISSOLVED SULFATE (SO4=) AND AT LEAST ONE SOLUBLE SILICOFLUORIDE-CONTAINING COMPOUND IN AN AMOUNT SUFFICIENT TO PRODUCE A CONCENTRATION OF 1.0 TO 11.4 G./1. OF DISSOLVED SILICOFLUORIDE (DIF6=), SAID BATH BEING FREE OF COMPOUNDS ACTING TO SUPPRESS THE CONCENTRATIONS OF DISSOLVED SULFATE AND SILICOFLUORIDE, THE SUM OF SAID DISSOLVED SULFATE AND SILICOFLUORIDE BEING IN THE RANGE OF 1.5 TO 11.7 G./1., AND SAID SUM OF DISSOLVED SULFATE AND SILICOFLUORIDE VARYING WITH THE CRO3 CONCENTRATION IN THE MANNER DEFINED BY THE AREA ABCD OF THE GRAPH SHOWN IN THE ACCOMPANYING DRAWING. 